Process for cross-linking a polymer

ABSTRACT

Process for cross-linking a polymer or a resin with the help of a peroxide, in the presence of a co-agent, the co-agent used being a compound according to formula 1, in which n is higher than or equal to 2, R 1  is an aliphatic or aromatic group and the R 2  and R 3  groups, independently of each other, can be a hydrogen atom, an aliphatic or an aromatic group. The polymer used preferably is EPDM or CSM.

[0001] The invention relates to a process for cross-linking a polymer ora resin with the help of a peroxide, in the presence of a co-agent

[0002] Polymers and resins that are crosslinked are for instanceelastomers and thermosetting resins. Before the cross-linking, thesepolymers and resins are liquid at room temperature or at an elevatedtemperature, so that they can be processed into moulded parts, forinstance by casting, compression moulding, injection moulding,extrusion, etc. When moulded, the polymer or the resin is cross-linked,whereby the polymer or the resin obtains its final properties as desiredin view of the application, Thus, many cross-linked thermosetting resinsexhibit a high stiffness, also at very high temperatures. Cross-linkedelastomers behave rubber-like, as manifested for instance in arelatively low stiffness and a high, reversible elongation.

[0003] Depending on the type of polymer or resin it is possible toeffect the cross-linking by means of a free-radical reaction, mostlywith the help of peroxide. In order to enhance the efficiency of such areaction it is often carried out in the presence of a co-agent. Due tothe presence of a co-agent a network having a higher cross-link densityis obtained, with the same amount of peroxide supplied. It is alsopossible to use a smaller amount of peroxide and yet obtain a comparablecross-link density. A co-agent is a compound which in the cross-linkingprocess is integrated into the polymer network. A co-agent mostly hastwo or more carbon-carbon double bonds. Examples of the most commonlyapplied co-agents are triallyl cyanurate (TAC), polybutadiene,trimethylolpropane-tris-methacrylate (TRIM) and metaphenylenebis(maleimide) (BMI). Drawbacks of the known co-agents are for instancethat they still lack efficiency or that, while being efficient, theyalready react at a too low temperature. An example of a co-agent whichoften exhibits a less than efficient cross-linking reaction is TAC. Theuse of BMI as a co-agent is known from W. Hofman's ‘Rubber TechnologyHandbook’, (Hanser, 1996), page 107, lines 8-9. When bismaleimides areused as co-agent, the cross-linking reaction is mostly efficient, butthe cross-linking reaction rate is too high at low temperatures. As aresult, premature cross-linking will occur during the processing intothe moulded part, and because of the accompanying polymer viscosityincrease, processing of the polymer into the moulded part is notpossible or is detective.

[0004] The aim of the invention is to provide a process forcross-linking a polymer or a resin with the help of a peroxide, in thepresence of a co-agent, whereby a highly efficient cross-linkingreaction is obtained while nevertheless no or hardly any prematurecross-linking occurs during the processing of the polymer or the resininto a moulded part.

[0005] Surprisingly, such a process is obtained due to a compoundaccording to formula 1:

[0006] being used as co-agent,

[0007] where n is higher than or equal to 2,

[0008] R¹ is an aliphatic or aromatic group and the R² and R³ groups,independently of each other, are a hydrogen atom, an aliphatic or anaromatic group.

[0009] With the process according to the invention a highly efficientcross-linking reaction is obtained, while nevertheless no or hardly anypremature cross-linking occurs during the processing of the polymer intoa moulded part A further advantage is the good miscibility of thecompounds according to formula 1 with many polymers.

[0010] Another further advantage is that the compounds according toformula 1 can be prepared in a simple manner.

[0011] Yet another further advantage is that the polarity of the polymerobtained after cross-linking can be increased with compounds accordingto formula 1. This enables for instance the oil resistance of non-polarelastomers to be improved.

[0012] From U.S. Pat. No. 3,502,542 a process is known for bonding metalto a rubbery elastomer, comprising the steps of preparing a mixturecomprising an elastomer and an acidic compound and curing said elastomerby introducing free radicals while contacting said metal and saidelastomer. As acidic compounds in a long list for example Carboxyl;lcacids comprising more than one N-maleimido or N-maleamic acid radicalsare mentioned. The group according to formula 1 is not disclosed.However N,N′-(1,3-phenylene)di-maleamic acid is mentioned. U.S. Pat. No.3,502,542 is silent about efficiency of the cross-linking reaction. Amethod of binding an elastomer to a metal substrate by usingN,N′-(1,3-phenylene)di-maleamic acid is therefore excluded fromprotection.

[0013] From JP-A-901 2542 it is known to crosslink a mixture comprisinga bismaleiimide and a bismaleiimide acid ester. However nothing is saidabout the crosslink efficiency. A process for cross-linking a mixturecomprising a bismaleiimide and a bismaleiimide acid ester are thereforeexcluded from protection,

[0014] From U.S. Pat. No. 4,278,586 a method of Improving the adhesionbetween a polymer composition and a fiber is disclosed, by incorporatinga bismaleiimide acid in the polymer composition. No peroxidecross-linking however is applied.

[0015] From EP-A619343 a method is known for coating metal pipes with apolymer composition. To prepare the composition a modified polypropyleneis added to a crosslinked polyolefin. The polypropylene is modified byfor example an isopherone bismaleamic acid. However the bismaleamic acidis grafted onto the polypropylene and no cross-linking reaction takesplace in the presence of the bismaleamic acid. If R¹ of the compoundaccording to formula 1 is an aromatic group, then R¹ is preferably aphenyl group. If R¹ is an allphatic group, then R¹ preferably contains1-12 C atoms. Still more preferably, R¹ is an aliphatic group with twoor six carbon atoms. Most preferably, R¹ is an aliphatic group with sixcarbon atoms. The value of n preferably is 2-4. More preferably, thevalue of n is 2. R² and R³ are preferably each a hydrogen atom.

[0016] The double carbon-carbon bond shown in formula 1 can be either inthe cis or in the trans configuration. Preferably, the doublecarbon-carbon bond is in the cis configuration. Examples of suitablecompounds according to formula 1 are phenylene bis(maleamide acid)(FBMA) and hexamethylene bis(maleamide acid) (HMBMA). Preferably,hexamethylene bis(maleamide add) is used.

[0017] The compounds according to formula I can be prepared for instanceby reacting a compound having at least 2 amine groups with an equimolaramount of α-β unsaturated dicarboxylic acid, anhydride, ester orsemi-ester.

[0018] The reaction can be carried out for instance by dissolving thecompound with the amine groups and combining the carboxylic acid, theanhydride, the ester or the semi-ester with stirring, keeping thetemperature between 0 and 30° C. Suitable solvents are for instancetetrahydrofuran and methyl-tert-butyl ether (MTBE).

[0019] In principle all polymers and resins that can be cross-linkedwith the help of a peroxide can be used in the process according to theinvention, These are often polymers or resins without double bonds inthe main chain. However, it is also possible to use polymers or resinswith double bonds in the main chain.

[0020] By cross-linking the polymer or resin is understood that at leastpart of the polymer or resin forms a three-dimensional polymer network.This means that the part of the polymer or resin forming thethree-dimensional network, also mentioned gel fraction, cannot any morebe dissolved in a suitable solvent for the polymer or resin. The solfraction is the part of the polymer or resin still being dissolved inthe solvent Together the gel fraction and the sol fraction form 100% ofthe polymer or resin.

[0021] Preferably the gel fraction is at least 50%, more preferably 75%,still more preferably at least 95%, most preferably at least 99%.

[0022] Examples of thermosettng resins which are suitable for use in theprooess according to the invention are unsaturated polyester resins,alkyd resins, acrylate resins.

[0023] The process according to the invention gives good results whenapplied for the cross-linking of elastomers. Examples of elastomerswhich are very suitable to be used in the process according to theinvention are chlorosulphonated polyethene (CSM), acrylate rubbers(ACM), chlorinated polyethene (CM), hydrogenated nitrilbutadiene rubber(H—NBR), silicone rubber (QM), fluororubber (FKM), polyethenevinylacetate (EVA), elastomers obtained by the polymerization of etheneand an alpha olefin (for instance EPM) and elastomers obtained by thepolymerisation of ethers, an alpha olefin and a non-conjugated polyene(for instance EPDM).

[0024] The process according to the invention gives very good resultswhen using elastomers obtained by the polymerization of ethene and anα-olefin (EPM). As α-olefin an α-olefin with 3-10 carbon atoms can forinstance be used; examples are propane, butene, hexene, octene etc.Preferably, propene is used.

[0025] The process according to the invention also gives very goodresults when using elastomers obtained by polymerization of ethene, ana-olefin and a non-conjugated polyene (EPDM).

[0026] As α-olefin is used for instance an α-olefin with 3-10 carbonatoms; examples are propane, butane, hexene, octene etc. Preferably,propene is used.

[0027] The ethene to α-olefin weight ratio is preferably between 90/10and 20/80. Preferably, the ethene to α-olefin weight ratio is between70/30 and 40/60.

[0028] Examples of non-conjugated polyenes to be applied in the EPDM are5-ethylidene-2-norbornene, 5-vinyl-2-norbornene, dicyclopentadiene or1,4 hexadiene or mixtures thereof. The preparation of EPDM is known tothe person skilled in the art. EPDM can for instance be prepared bypolymerization with the help of a Ziegler-Natta catalyst or ametallocene catalyst.

[0029] The process according to the invention also gives very goodresults when chlorosulphonated polyethene (CSM) is used. CSM can forinstance be prepared by means of UV irradiation of polyethene in aninert chlorinated solvent at 70-75° C. in the presence of gaseouschlorine and sulphur dioxide. The preparation of CSM is known to theperson skilled in the art

[0030] A yet further improved process according to the invention isobtained if the cross-linking of the CSM is carded out in the presenceof a peroxide, a co-agent and an amine accelerator.

[0031] The polymer composition contains for instance 7*10−4-2.2*10⁻¹ molof the amine accelerator per kilogram of CSM. Preferably, the polymercomposition contains 3.5*10⁻²-1.5*10⁻¹ mol of the amine accelerator perkilogram of CSM.

[0032] Preferably, tertiary amines are used as amine accelerator. Morepreferably, use is made of hexamethylene tetramine (HMTA).

[0033] Prior to carrying out the process according to the invention, acomposition is mostly prepared which contains the polymer or the resin,the peroxide and the co-agent, by mixing the polymer or the resin, theperoxide and the co-agent at a temperature below the temperature atwhich the polymer or the resin is crosslinked. The peroxide and theco-agent can be mixed simultaneously with the polymer or the resin, butit is also possible to mix the co-agent and the peroxide successivelywith the polymer or the resin. It may be of advantage for instance, tomix the peroxide with the polymer or the resin, which is previouslymixed with the co-agent, shortly prior to the processing the compositioninto a moulded part. In this way the polymer or the resin is preventedfrom cross-linking already during storage subsequent to the mixing andprior to processing of the composition into the moulded part.

[0034] Examples of suitable peroxides are di-tert-butylperoxide,bis(tert-butylperoxyisopropyl)benzene (Perkadox™ 14/40 MB, Akzo Chemie),dicumylperoxide, butyl 4,4-bis(tert-butylperoxy)valerate,2,5-bis(tert-butylperoxy)-2,5-methylhexane,2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyn, tert-butyl3-isopropenylcumylperoxide.

[0035] The composition containing the polymer or the resin contains forinstance 2.9*10⁻³-3*10⁻¹ mol peroxide per kilogram of polymer or resin.Preferably, the composition contains 1.1*10⁻²-1.5*10⁻¹ mol peroxide perkilogram of polymer or resin. More preferably, the composition contains1.4*10⁻²-8.9*10⁻² mol peroxide per kilogram of polymer or esin. At thisrelatively low peroxide concentration the invention is highly effective.

[0036] The composition contains for instance 3.1*10⁻²-4.8*10⁻¹ molco-agent per kilogram of polymer or resin. Preferably, the compositioncontains 6.2*10⁻²-3.2* 10⁻¹ mol co-agent per kilogram of polymer orresin. More preferably, the polymer composition contains6.2*10⁻²-1.6*10⁻¹ mol co-agent per kilogram of polymer or resin.

[0037] The processing of the composition containing the polymer or theresin, the peroxide and the cogent into a moulded part usually takesplace at a temperature higher than room temperature. Raising thetemperature causes the viscosity of the composition to decline and as aresult the moulding is made easier. That is why it is Important to carryout the moulding at a relatively high temperature. At the same time,however, it is also important that the cross linking reaction does nottake place, or only to a minor extent, prior to the moulding. After themoulding the temperature of the composition is usually increasedfurther, so that the peroxide decomposes and the cross-linking reactiontakes place. In this relation it is important that the cross-linkingreaction takes place efficiently. So during the moulding the reactionshould not occur or only to a minor extent, while after the moulding thereaction should take place efficiently. It has been found that due tothe use of the co-agent according to formula 1 in the process accordingto the invention an efficient cross-linking reaction is obtained whilenevertheless no or hardly any premature cross-linking occurs during theprocessing of the polymer or resin into a moulded part.

[0038] The process according to the invention will now be illustrated bymeans of the following non-limiting examples.

[0039] A maleic anhydride solution was prepared by dissolving 15.0 grams(0.153 mol) of maleic anhydride in 250 ml of tetrahydrofuran withstirring at room temperature. In 5 minutes a solution of 8.27 grams(0.076 mol) of metaphenylene diamine in 250 ml of tetrahydrofuran wasadded dropwise to the maleic anhydride solution. A yellow suspension wasobtained. Next the whole was stirred for 10 minutes, after which theyellow suspension was filtered and washed with 50 ml of tetrahydrofuran.The residue was a yellow solid substance in powder form, 23.1 grams(0.076 mol). Part of this yellow solid substance was dissolved indimethyl sulphoxide (DMSO) and it was then Identified as FBMA by meansof 1 H-NMR measurements.

[0040] The preparation of the test material took place in an internalmixer (Brabender Plasticorder, 50 cc). The composition data of the testmaterial are presented in Tables 1 and 4. Keltan™ 720 (EPDM, DSM) wasfirst introduced into the kneader, at a temperature of 75° C., followedby the other ingredients, except for the peroxide and the co-agent. Thewhole was kneaded for 1 minute at 40 rpm and 75° C. Then the peroxideand the co-agent were added to the composition, which subsequently waskneaded for 4 minutes under the same circumstances Then the testmaterial was pressed in 2 minutes at 60° C. to 2 mm thick sheets in aFonteijne press, after which the measurements were effected.

[0041] The test material obtained was tested according to ISO6502 andISO667. The ISO6502 test method was used to determine the cross-linkingefficiency under the curing conditions for the material. Themeasurements were performed with a Monsanto™ rheometer MDR 200^(E). Themeasurement took place at 180° C. in 1200 seconds (20 minutes). Amaximum and a minimum torque (N.m) were determined, as well the time atwhich 90% of the maximum torque had been reached (t90). The maximumtorque is a measure of the degree of cross-linking of the material. Thehigher the maximum torque, the higher the degree of cross-linking. Thet90 value is a measure of the rate of cross-linking. The lower t90, thehigher the rate at which the cross-linking takes place.

[0042] The test method according to ISO667 was used as a method ofmeasurement to determine the degree of cross-linking of the materialunder moulding conditions.

[0043] The measurements were carried out with the Monsanto™ Mooney MV2000E. The measurements took place at 125° C. in 3600 seconds (60minutes). The development of the viscosity shows whether prematurecross-linking of the material occurs. The viscosity is expressed inMooney units (MU), An increase in viscosity indicates the occurrence ofcross-linking. The initial viscosity and the minimum viscosity weremeasured.

[0044] In addition, the points in time were determined at which theviscosity had increased by 2, 5 and 35 Mooney units, respectively,relative to the minimum viscosity (t2, t5 and t35, respectively). Themeasured t2, t5 and t35 provide an indication of the cross-linkingreaction rate. The faster these points in time follow each other, thefaster the cross-linking takes place.

EXAMPLE I AND COMPARATIVE EXPERIMENTS A AND B Comparison ofhexamethylene bis(maleamide acid) (HMBMA) and triallyl cyanurate (TAC)

[0045] TABLE 1 Composition of test material Ingredient From I A BKeltan ™ 720 (EPDM) DSM 100 100 100 ZnO active Bayer AG 5 5 5 Stearicacid HCHM SA/18, 0.5 0.5 0.5 Simel S. p. A N-550 carbon black Sterling SO, 70 70 70 Cabot B. V. Mikrosohl ™-M40 Ver. Kreidewerke 40 40 40whiting Dammann Tudalen ™ B 8014, Klaus Dahleke KG 70 70 70 paraffinicoil Perkadox ™ 14/40 MB Akzo Chemie B. V. 2 2 2 HMBMA 5 TAC-70 DegussaAG 5 Total phr 292.5 287.5 292.5

[0046] The composition of the test materials used in example I and incomparative experiments A and B is shown in table 1. The test materialof example I was prepared according to the process of the invention andcontains HMBMA as co-agent. Comparative experiment A relates to a testmaterial prepared without a co-agent, while for experiment B TAC wasused as co-agent TABLE 2 Results of measurements under curing conditionsISO 6502 I A B Minimum torque N.m 0.07 0.07 0.07 Maximum torque N.m 0.520.29 0.35 t90 s 352 280 400

[0047] Comparing in table 2 the results of example I with comparativeexperiments A (no co-agent) and B (TAC as co-agent) it is seen that themaximum torque of example I is higher than in comparative experiments Aand B. A higher maximum torque is indicative of a higher degree ofcross-linking of the test material. In addition, the time at which 90%of the maximum torque has been reached is shorter in example I than incomparative experiment B, which indicates that the cross-linkingreaction rate in example I is higher. So it can be concluded that theuse of HMBMA as co-agent results in a higher efficiency of thecross-linking reaction than TAC. TABLE 3 Results of measurements undermoulding conditions ISO 667 I A B Initial Mooney MU 32.7 32.7 32.0Minimum Mooney MU 21.3 20.0 19.8 t2 s — — — t5 s — — — t35 s — — —

[0048] From the measurement results in table 3 it appears that in noneof the examples cross-linking occurs in the test period. This indicatesthat there is no premature cross-linking under moulding conditions, i.e.when the material is moulded.

EXAMPLE II AND COMPARATIVE EXPERIMENTS A AND C Comparison ofhexamethylene bis(maleamide acid) (HMBMA) andtrimethylolpropane-tris-acrylate (TRIM)

[0049] TABLE 4 Composition of test material Ingredient From II A CKeltan ™ 720 (EPDM) DSM 100 100 100 ZnO active Bayer AG 5 5 5 Stearioacid HCHM SA/18, Simel 0.5 0.5 0.5 S. p. A N-550 carbon black Sterling SO, 70 70 70 Cabot B. V. Mikrosohl ™-M40 Ver. Kreidewerke 40 40 40whiting Dammann Tudalen ™ B 8014, Klaus Dahleke KG 70 70 70 paraffinicoil Perkadox ™ 14/40 MB Akzo Chemie B. V. 2 2 2 HMBMA 10 Sartomer ™ 350(TRIM) Cray Valley 10 Totaal phr 297.5 287.5 297.5

[0050] The composition of the test materials used in example II and incomparative experiments A and C is shown in table 4. The teat materialof example I was prepared according to the process of the invention andcontains HMBMA as co-agent Comparative experiment A relates to a testmaterial prepared without a co-agent, while for experiment C TRIM wasused as co-agent. TABLE 5 Results under curing conditions ISO 6502 II AC Minimum torque N.m 0.08 0.07 0.06 Maximum torque N.m 0.58 0.29 0.58t90 s 448 280 368

[0051] From table 5 it appears that in example II (HMBMA as co-agent)and comparative experiment C (TRIM as co-agent) the same maximum torqueis reached. This indicates that the two co-agents give a comparablecross-linking efficiency under curing conditions, TABLE 6 Results undermoulding conditions ISO 667 II A C Initial Mooney MU 33.8 32.7 26.9Minimum Mooney MU 22.0 20.0 16.6 t2 s — — 1292 t5 s — — 1340 t35 s — —1548

[0052] From table 6, however, it appears that the test material ofcomparative experiment C (TRIM as co-agent) begins to cross-link underthe moulding conditions, while in example II (HMBMA as co-agent), as incomparative experiment A (no co-agent), there is no cross-linking yet.

[0053] Further, in experiment C the relatively small difference in timebetween the moment at which the viscosity has increased by 2 unitsrelative to the minimum viscosity under the moulding conditions and themoment at which the viscosity has increased by 35 units relative to theminimum viscosity under the moulding conditions indicates that, once thecross-linking reaction has started, cross-linking takes place rapidlyunder the moulding conditions. This indicates that when HMBMA is used asco-agent in the moulding of the material, there is hardly anycross-linking, in contrast to the effect of TRIM being used as co-agent.A low degree of cross-linking under the moulding conditions isdesirable.

1. Process for cross-linking a polymer or a resin with the help of aperoxide, in the presence of a co-agent, characterized in that aco-agent according to formula 1

is used, in which n is higher than or equal to 2, R¹ is an allphatic oraromatic group and the R² and R³ groups, independently of each other,can be a hydrogen atom, an aliphatic or an aromatic group, excluding aprocess for cross-linking a composition comprising a bismaleiimide resinand a bismaleiimide acid ester and excluding a process for binding anelastomer to a metal substrate by using N,N′-(1,3-phenylene)di-maleamicacid.
 2. Process according to claim 1, characterized in that R¹ is aphenyl group or a C1-C12 alkyl group.
 3. Process according to any one ofclaims 1-2, characterized in that R¹ is an aliphatic group with two orsix carbon atoms.
 4. Process according to any one of claims 1-3,characterized in that n=2.
 5. Process according to any one of claims 14,characterized in that R² is a hydrogen atom.
 6. Process according to anyone of claims 1-5, characterized in that R³ is a hydrogen atom. 7.Process according to any one of claims 1-6, characterized in that1.4*10⁻²-8.9*10⁻² mol peroxide per kilogram of polymer or resin is used.8. Process according to any one of claims 1-7, characterized in that thepolymer is an elastomer.
 9. Process according to claim 8, characterizedin that the elastomer is a polymerization product of ethene, an alphaolefin and a non-conjugated polyene.
 10. Process according to claim 8,characterized in that the elastomer is a chlorosulphonated polyethene.11. Process according to claim 8, characterized in that the elastomer isa chlorosulphonated polyethene, without the exclusions of claim
 1. 12.Process for cross-linking an elastomer with the help of a peroxide, inthe presence of a co-agent, characterized in that a co-agent accordingto formula 1

is used, in which n is higher than or equal to 2, R¹ is an aliphatic oraromatic group and the R² and R³ groups, independently of each other,can be a hydrogen atom, an aliphatic or an aromatic group excluding aprocess for binding an elastomer to a metal substrate.
 13. Process forcross-linking a resin with the help of a peroxide, in the presence of aco-agent, characterized in that a co-agent according to formula 1

is used, in which n is higher than or equal to 2, R¹ is an aliphatic oraromatic group and the R² and R³ groups, independently of each other,can be a hydrogen atom, an aliphatic or an aromatic group, excluding aprocess for cross-linking a composition comprising a bismaleiimideresin.